Silver catalysts are well known in the art as useful in the production of ethylene oxide by the vapor phase oxidation of ethylene with molecular oxygen at elevated temperature. Much is taught in the literature concerning preparation of such catalysts and a variety of methods have been suggested which produce small particles of silver and relatively uniform distribution on a suitable support.
Generally, the support is impregnated or coated with a solution of the salt of the metal or metals to be employed. This is followed by drying and a subsequent reduction. Although the reducing step can be a thermal reduction in the presence of air or an inert gas (U.S. Pat. No. 2,709,123), hydrogen and hydrazine (U.S. Pat. No. 3,575,888) have also been employed for the reduction. Reducing agents in the form of organic compounds have been added to the catalyst, either by simultaneous application or subsequent addition to the catalytic component. Also reducing compounds have been known to be employed by incorporating them into the support prior to the addition of the catalytic component.
In one early patent (U.S. Pat. No. 2,920,052) a silver salt of an organic acid, e.g. silver oxalate, was used for the silver source and the reduction was carried out by wetting with diethylene glycol and heating to reduce the silver salt and to remove the excess glycol. The use of silver salts of carboxylic acids was also taught in an earlier patent (U.S. Pat. No. 2,446,132) and glycols were suggested as reducing agents (U.S. Pat. No. 3,563,914). In one procedure (U.S. Pat. No. 3,702,259) silver carboxylates are employed with an "organic amine solubilizing/reducing" agent or ammonia may be used with the organic amine. In another similar procedure the impregnation is followed by a methanol wash prior to reduction (U.S. Pat. No. 4,102,820).
Other nitrogen-containing compounds, e.g. polyacrylonitrile (U.S. Pat. No. 3,892,679), an alkanolamine or an acid amide (U.S. Pat. No. 4,248,740) have also been employed. While the solvent used to apply the reducing agent is frequently water as in U.S. Pat. No. 3,702,259 above, organic solvents have also been employed, as for example, ethylene carbonate, dimethyl formamide and dimethyl sulfoxide as solvents for the polyacrylonitrile in U.S. Pat. No. 3,892,679, above.
Supports taught in the art are silica, alumina or other inert low surface area support materials. For example .alpha.-alumina is disclosed as a useful support in U.S. Pat. Nos. 3,305,492 and 3,172,893.
Various methods are used for incorporating the silver salt and include vacuum-impregnation (U.S. Pat. No. 3,702,259) which apparently provides a more complete coverage of the support surfaces.
The present invention is a new combination of old steps in which the silver salt of an organic acid, e.g. silver acetate, is dissolved in aqueous ammonium hydroxide. This solution is used to impregnate the support. The impregnated support is dried at low temperature, and then contacted with a polyhydroxy compound, e.g. monoethylene glycol, and reacted therewith to reduce the silver salt. This is followed by a water-wash and a final drying at low temperature (about 50.degree. to about 150.degree. C.).